Reliable, modular, production quality narrow-band high rep rate F2 laser

ABSTRACT

The present invention provides a reliable modular production quality excimer laser capable of producing 10 mJ laser pulses in the range of 1000 Hz to 2000 Hz or greater. Replaceable modules include a laser chamber; a pulse power system comprised of three modules; an optical resonator comprised of a line narrowing module and an output coupler module; a wavemeter module, an electrical control module, a cooling water module and a gas control module. Important improvements have been provided in the pulse power unit to produce faster rise time and improved pulse energy control. These improvements include an increased capacity high voltage power supply with a voltage bleed-down circuit for precise voltage trimming, an improved communication module that generates a high voltage pulse from the capacitors charged by the high voltage power supply and amplifies the pulse voltage 23 times with a very fast voltage transformer having a secondary winding consisting of a single four-segment stainless steel rod. A novel design for the compression head saturable inductor greatly reduces the quantity of transformer oil required and virtually eliminates the possibility of oil leakage which in the past has posed a hazard.

This application is a Continuation-In-Part of Ser. No. 09/271,041,Reliable, Modular, Production Quality Narrow-Band High Rep Rate ArFExcimer Laser filed Mar. 17, 1999; Ser. No. 09/041,474. Reliable,Modular, Production Quality Narrow Band KrF excimer Laser, filed Mar.11, 1998; Ser. No. 08/995,832, Excimer Laser Having Pulse Power Supplywith Fine Digital Regulation, filed Dec. 22, 1997; Ser. No. 08/896,384Wavelength Reference for Excimer Laser, filed Jul. 18, 1997; Ser. No.08/939,611, Protective Overcoat for Replicated Diffraction Gratings,filed Sep. 29, 1997; Ser. No. 08/947,474, Narrow Band Excimer Laser,filed Oct. 10, 1997; Ser. No. 09/034,870, Pulse Energy Control forExcimer Laser, filed Mar. 4, 1998; Ser. No. 09/082,139, Narrow BandExcimer Laser with Gas Additive, filed May 20, 1998; Ser. No.09/157,067, Reliable Modular Production Quality Narrow Band High RepRate Excimer Laser, filed Sep. 18, 1998; Ser. No. 09/162,341, LineNarrowing Apparatus with High Transparency Prism Beam Expander filedSep. 28, 1998; Ser. No. 09/165,593, Wavelength System for an ExcimerLaser filed Oct. 2, 1998; Ser. No. 09/206,526, Wavelength Reference forLaser, filed Dec. 7, 1998; Ser. No. 09/211,825, High Pulse Rate PowerSystem with Resonant Power Supply filed Dec. 15, 1998; Ser. No.09/217,340 Durable Etalon Based Output Coupler filed Dec. 21, 1998 allof which are incorporated herein by reference. This invention relates tolasers and in particular to narrow-band ArF excimer lasers.

BACKGROUND OF THE INVENTION KrF Excimer Lasers

Krypton-Fluoride (KrF) excimer lasers are currently becoming theworkhorse light source for the integrated circuit lithography industry.The KrF laser produces a laser beam having a narrow-band wavelength ofabout 248 nm and can be used to produce integrated circuits withdimensions as small as about 180 nm. The Argon Fluoride (ArF) excimerlaser is very similar to the KrF laser. The primary difference is thelaser gas mixture and a shorter wavelength of the output beam.Basically, Argon replaces Krypton and the resulting wavelength of theoutput beam is 193 nm. This permits the integrated circuit dimensions tobe further reduced to about 120 nm. F₂ lasers have long been recognizedas the successor to the KrF and ArF lasers in the integrated circuitlithography industry since the F₂ beam at 157 nm permits a substantialimprovement in pattern resolution. These F₂ lasers can be very similarto the KrF and ArF excimer lasers and with a few modifications it ispossible to convert a prior art KrF or ArF laser to operate as an F₂laser. A typical prior-art KrF excimer laser used in the production ofintegrated circuits is depicted in FIG. 1 and FIG. 2. A cross section ofthe laser chamber of this prior art laser is shown in FIG. 3. A pulsepower system 2 powered by high voltage power supply 3 provideselectrical pulses to electrodes 6 located in a discharge chamber 8.Typical state-of-the art lithography lasers are operated at a pulse rateof about 1000 Hz with pulse energies of about 10 mJ per pulse. The lasergas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the restneon which functions as a buffer gas) at about 3 atmospheres iscirculated through the space between the electrodes at velocities ofabout 1,000 inches per second. This is done with tangential blower 10located in the laser discharge chamber. The laser gases are cooled witha heat exchanger 11 also located in the chamber and a cold plate (notshown) mounted on the outside of the chamber. The natural bandwidth ofthe excimer lasers is narrowed by line narrowing module 18. Commercialexcimer laser systems are typically comprised of several modules thatmay be replaced quickly without disturbing the rest of the system.Principal modules include:

Laser Chamber Module,

Pulse Power System with: high voltage power supply module,

commutator module and high voltage compression head module,

Output Coupler Module,

Line Narrowing Module,

Wavemeter Module;

Computer Control Module,

Gas Control Module,

Cooling Water Module

Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supportedin this prior art embodiment by anode support bar 44 which is shown incross section in FIG. 3. Flow is clockwise in this view. One corner andone edge of anode support bar 44 serves as a guide vane to force airfrom blower 10 to flow between electrodes 6A and 6B. Other guide vanesin this prior art laser are shown at 46, 48 and 50. Perforated currentreturn plate 52 helps ground anode 6B to the metal structure of chamber8. The plate is perforated with large holes (not shown in FIG. 3)located in the laser gas flow path so that the current return plate doesnot substantially affect the gas flow. A peaking capacitor comprised ofan array of individual capacitors 19 is charged prior to each pulse bypulse power system 2. During the voltage buildup on the peakingcapacitor, two preionizers 56 weakly ionize the lasing gas betweenelectrodes 6A and 6B and as the charge on capacitors reach about 16,000volts, a discharge across the electrode is generated producing theexcimer laser pulse. Following each pulse, the gas flow between theelectrodes of about 1 inch per millisecond, created by blower 10, issufficient to provide fresh laser gas between the electrodes in time forthe next pulse occurring one millisecond later.

In a typical lithography excimer laser, a feedback control systemmeasures the output laser energy of each pulse, determines the degree ofdeviation from a desired pulse energy, and then sends a signal to acontroller to adjust the power supply voltage so that energy ofsubsequent pulses are close to the desired energy. In prior art systems,this feedback signal is an analog signal and it is subject to noiseproduced by the laser environment. This noise can result in erroneouspower supply voltages being provided and can in turn result in increasedvariation in the output laser pulse energy.

These excimer lasers are typically required to operate continuously 24hours per day, 7 days per week for several months, with only shortoutages for scheduled maintenance. One problem experienced with theseprior-art lasers has been excessive wear and occasional failure ofblower bearings.

A need exists in the integrated circuit industry for a modular,reliable, production line quality F₂ laser in order to permit integratedcircuit resolution not available with KrF and ArF lasers.

SUMMARY OF THE INVENTION

The present invention provides a reliable, modular, production qualityF₂ excimer laser capable of producing, at repetition rates in the rangeof 1,000 to 2,000 Hz or greater, laser pulses with pulse energiesgreater than 10 mJ with a full width half, maximum bandwidth of about 1or less. Preferred embodiments of the present invention can be operatedin the range of 1000 to 4000 Hz with pulse energies in the range of 10to 5 mJ with power outputs in the range of 10 to 40 watts. Using thislaser as an illumination source, stepper or scanner equipment canproduce integrated circuit resolution of 0.1 μm or less. Replaceablemodules include a laser chamber and a modular pulse power system.

Important improvements over prior art excimer lasers have been providedin the pulse power unit to produce faster charging. These improvementsinclude an increased capacity high voltage power supply, an improvedcommunication module that generates a high voltage pulse from capacitorscharged by the high voltage power supply and amplifies the pulse voltageabout 23 times with a very fast voltage transformer having a secondarywinding consisting of a single four-segment stainless steel rod. A noveldesign for the compression head saturable inductor (referred to hereinas a “pots and pans” design) greatly reduces the quantity of transformeroil required and virtually eliminates the possibility of oil leakagewhich in the past has posed a hazard.

Improvements in the laser chamber permitting the higher pulse rates andimproved performance include the use of a single preionizer tube.

In a preferred embodiment the laser was tuned to the F₂ 157.6 nm lineusing a set of two external prisms. In a second preferred embodiment thelaser is operated broad band and the 157.6 nm line is selected externalto the resonance cavity. In a third preferred embodiment a line width of0.2 pm is provided using injection seeding.

Other embodiments of the present invention include ceramic bearings.Optionally magnetic bearings may be utilized. Reaction forces on thebearings may be reduced by providing an aerodynamic contour on the anodesupport bar. Other improvements include use of acoustic baffles forlaser chambers producing disruptive acoustic shock waves.

Preferable the intracavity beam line and the output beam line fullysealed and nitrogen purged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a prior art commercial excimer lithography laser.

FIG. 2 is a block diagram showing some of the principal elements of aprior art commercial excimer lasers used for integrated circuitlithography.

FIG. 3 is a drawing of the laser chamber of the FIG. 2 laser.

FIG. 4 is a drawing of a preferred embodiment of the present invention.

FIG. 5 is a drawing showing a blower drive unit including magneticbearings.

FIGS. 6 and 6A are cross section drawings of laser chambers of preferredembodiments of the present invention.

FIG. 7 is a drawing showing features of a preferred preionizer tube.

FIG. 8A is a block diagram of a pulse power system of the preferredembodiment of the present invention.

FIG. 8B is a simplified circuit diagram of the above preferredembodiment.

FIG. 8C is a combination block diagram, circuit diagram of a highvoltage power supply which is part of the above preferred embodiment.

FIG. 8D is a prospective assembly drawing of a pulse transformer used inthe above preferred embodiment.

FIG. 8E is a drawing of a primary winding of a pulse transformer used inthe above preferred embodiment.

FIGS. 8F1, 8F2 and 8F3 are time line charts showing pulse compressionusing the above preferred embodiment.

FIGS. 8G1 and 8G2 are drawing showing two views of a saturable inductor.

FIGS. 8H1 and 8H2 shows the mounting of a compression head in apreferred embodiment.

FIGS. 9A and 9B are drawings describing a preferred heat exchangerdesign.

FIGS. 10A through 10F are graphs of test data taken during experimentswith a prototype F₂ laser.

FIGS. 11A and 11B show two preferred F₂ system configurations.

FIGS. 12A through 12E show various anode support bar designs.

FIG. 13 describes a preferred enclosure cooling system.

FIGS. 14A, 14B and 14C show preferred blower blade structure designs.

FIGS. 15A and 15B show F₂ laser performance data.

FIG. 16 shows a big manifold gas supply system.

DETAILED

DESCRIPTION OF PREFERRED EMBODIMENTS

First Preferred Embodiment

A preferred embodiment of the present invention can be described byreference to the drawings.

Modular Laser Design

A front view of a preferred embodiment of the present invention is shownin FIG. 4 respectively. This drawing emphasizes the modular nature oftheir particular invention which allows very quick replacement ofmodules for repair, replacement and maintenance. The principal featuresof this embodiment are listed below corresponding to the referencenumbers shown on FIG. 4.

201 Laser enclosure

202 Gas module

203 Cooling water supply module

204 AC/DC distribution module

205 Control module

206 Line narrowing module

207 Compression head

208 High voltage pulse power supply module

209 Commutator module for pulse power supply

210 Metal fluoride trap

211 Laser chamber

213 Wavemeter module

214 Automatic shutter

216 Output coupler

217 Blower motor

218 Metal fluoride trap power supply

219 Status lamp

220 24 volt power supply

221 Chamber window

222 Gas control flexible connection

224 Vent box

Preferred Embodiment

A preferred embodiment of the present invention is an improved versionof the laser described in FIGS. 1, 2 and 3. This preferred embodimentincludes the following improvements:

1) A single tube larger preionizer replaces the prior-art combination ofa two-tube preionizer to provide improved efficiency, betterpreionization and improved laser gas flow between the electrodes;

2) A silicon-free fan blade which may be a one-piece machined blade;

3) The solid-state pulse power system has been modified to producefaster rise time, providing more consistent pulses, and improved laserefficiency at higher voltages;

4) More precise control of the charging voltage of the pulse powersystem;

5) A computer controller programmed with a new algorithm providing amuch improved control of pulse energy and burst energy; and

6) Electrode spacing has been reduced to 10 mm.

Chamber Improvements Single Preionizer Tube

As shown in FIG. 6, a single larger preionizer tube 56A has replaced thetwo-preionizer tubes 56 shown in FIG. 3. The single tube preionizer isfabricated in accordance with the description in U.S. Pat. No.5,719,896, Issued Feb. 17, 1998, which is incorporated herein byreference. Applicants have discovered that one preionizer tube is notonly sufficient, but very surprisingly provides improved performanceover the two-preionizer design. In this embodiment the preionizer islocated upstream of the electrodes. Applicants have determined that theone tube preionizer improves in the pulse=to-pulse stability byproviding improved spatial stability of the discharge.

Referring now to FIG. 7, this preionizer utilizes an integrated tubedesign, having bushing element 180 with anti-tracking grooves 170incorporated therein as an integral component of the tube. The diameterof the rod portion 145 and the OD of the bushing portion 180 of thepreionizer is ½ inch. The inside conductor rod 146 has a diameter of{fraction (7/37)} inch and the connecting wire extending through thebushing section to make a ground connection is about {fraction (1/16)}inch diameter. Prior preionizer tube designs utilized a two-diameterdesign, with the rod portion at about ¼ inch diameter and the bushingsat about 1 inch diameter. This necessitated, for manufacturing purposes,a bonding process to join the bushing component with the tube component.The constant diameter, thicker tube design is contrary to conventionaldesign rules, which would predict a reduction in ionization due to lowercapacitances. In most designs, the tube thickness is dependent upon thedielectric strength of the material selected. Those skilled in the artwill recognize that the prior art conventional preionizer tube designtechnique is to select a material with the highest dielectric strengthand determining a wall thickness to match this capacity. For example, asapphire material is known to have a dielectric strength ranging from1200 volts/mil to 1700 volts/mil. Therefore, a dielectric thickness of0.035 inches thick, provides a safety factor of 2 if the laser operatesat 25 kV. This design yields a lower capacitance; however, the actualeffect of this reduced capacitance on laser operation was discovered tobe negligible, with a surprising increase in the measured geometricirradiation of the electrode gap. Because of the constant diameter,thicker tube wall, integral bushing design, a single piece of materialcan be machined to provide anti-tracking grooves 170. Because of thesingle piece construction, there is no need to use ultra-pure (i.e.,99.9%) polycrystalline translucent aluminum oxide ceramic, althoughApplicants continue to use the ultra-pure material. There is norequirement to perform the difficult surface polishing of tubegeometries in preparation for diffusion bonding to artificially createthe integral relationship between bushing 180 and tube 145. In fact, ithas been determined that high purity is not as important a property asporosity of the material. It has been found that the greater theporosity, the more the dielectric strength is reduced. As a result, acommercial grade ceramic, preferably with purity of at least 99.8% andlow porosity, such as that manufactured by Coors Ceramics Company underthe material No. AD-998E and having a dielectric strength of 300volts/mil may be used. Bushings 180, having anti-tracking grooves 170disposed therein, as previously described, act to prevent high voltagetracking axially along the surface of the tube from the cathode to theground plane 160.

As explained above, Applicants have discovered that a single preionizerworks dramatically better than two preionizers, and as explained abovethe first preferred embodiment places the single preionizer systemupstream of the electrodes. Applicants have also experimented with thesingle preionizer located downstream and has discovered that at certainblower speeds this arrangement produces substantially better pulseenergy stability than the upstream arrangement on the two tubearrangement.

High Efficiency Chamber

Improvements have been made to the chamber to improve the efficiency ofthe laser. A single piece cathode insulator 55A comprised of alumina,Al₂ O₃ insulates the cathode from the upper chamber structure as shownin FIG. 6A. In a prior art design, eight separate insulators were neededto avoid insulator cracking due to thermal expansion stress in theinsulator. This important improvement permitted the head portion of thechamber to be made shorter which significantly reduced the distancebetween cathode 83 the peaking capacitor 82. The individual capacitors54A forming the peaking capacitor array 82 were moved horizontally incloser to the cathode as compared to the prior art. To reduce thermalexpansion difference between the single piece insulator and the chamberstructure the upper chamber 8A was fabricated from ASTM A3C steel whichhas a coefficient of thermal expansion closer to Al₂ O₃ than aluminum.The bottom section 8B of chamber 8 is aluminum, but Applicants havedetermined that the difference in thermal expansion between ASTM A3Csteel and aluminum is not a problem. Both the steel and aluminum partsare nickel coated.

Prior art cathodes for commercial lithography lasers were typicallysupported by a cathode support bar 53 as shown in FIG. 3. In thispreferred embodiment, the cathode support bar was eliminated and thecathode 83 was made slightly thicker and mounted directly on the singlepiece insulator 55A. The cathode 83 is connected to the high voltageside 82A of peaking capacitor 82 by 15 feed through rods 83A andconnecting nuts 83B. In the preferred embodiment, a new anode supportbar 84A is substantially more massive than prior art anode support barsand comprises fins 84B located in the gas flow region. Both of thesefeatures minimize temperature variations of the anode.

Metal Seals

Applicants have discovered that prior art elastomer seals reacted withfluorine gas to produce contaminants in the laser gas which degradedlaser performance. A preferred embodiment of the present invention usesall metal seals to seal the laser chamber. The preferred metal seals aretin plated inconel 1718 seals.

Monel Current Return and Vanes

Applicants have also discovered that elements of stainless steel alsoreact with fluorine to produce contaminants in the laser gas. Therefore,in this preferred embodiment, prior art stainless steel current returnstructures and gas flow vanes have been replaced with monel currentreturns 250 and monel flow vanes 252 and 254.

Acoustic Baffles

Applicants have discovered that a significant cause of distortion of thequality of laser beams produced by narrow-band excimer lasers operatingat 2000 Hz or greater is acoustic shock waves created by the electricdischarge of one pulse which reflects from elements of chamber structureback to the space between the electrodes and distorts the laser beam ofthe next pulse occurring 0.5 millisecond later. An embodiment describedherein (see FIG. 6A) substantially minimizes this effect by providingangled, grooved acoustic baffles 63A and 64A on both sides of the laserchamber. These baffles absorb a portion of the acoustic energy andreflect a portion of the acoustic energy down into the lower region ofthe laser chamber away from the electrodes. In this preferredembodiment, the baffles consist of a machined metal structure withgrooves 0.1 mil wide, 0.3 mil deep spaced at 0.2 mil intervals; a 0.3mil deep groove is shown at 63 in baffle 60 in FIG. 6A. These baffleshave been shown by actual testing to substantially reduce pulse qualitydistortion caused by acoustic shock waves.

Applicants have also discovered that acoustic shock effects can beminimized by reducing streamers in the electric discharge. In fact, in apreferred embodiment of the present invention changes made in thechamber head (discussed above) and the new preionizer designed reducedacoustic shock so that acoustic baffles were not needed.

Fan Improvements

This preferred embodiment of the present invention includes majorimprovements in the prior art gas circulator which has greatly improvedlaser performance. These improvements are in the construction of a brazefree blower blade structure. A new non-symmetrical blade arrangementwhich greatly decreases resonance effects and improved bearings.

Silicon Free Fan Blade Structure

Applicants have discovered that a brazing material commonly used inblower blade construction was the primary source of SiF₆ in the laserchamber. This gas significantly degraded laser performance for KrFlasers but was a total disaster for ArF lasers and F₂ lasers. Applicantshave identified four solutions to this problem. First the bladestructure was machined in segments from a solid block of material (inthis case aluminum). Another solution was to cast the blade structure insegments. The segments then are welded together using electron beamwelding in which no new material is added. It is also feasible tofabricate the blade structure by joining blades to a frame structure butin this case the joining is by electron beam welding instead of theprior art brazing process. The fourth method is to join the blade to aframe structure using a soldering process using a silicon free solder.Aluminum 6061 is used as the base material for all of the componentpieces. These parts are then copper-plated in prelude to the solderingprocess. With all of the parts assembled, the fan is then solderedtogether using a low temperature solder, typically 91% tin (Sn) and 9%Zinc (Zn) in a vacuum furnace. This solder is chosen due to its lack ofsilicon and its ability to work with copper plated aluminum. Theassembled and soldered fan is then nickel-plated. This method ofconstruction yields a non-silicon fan that is inexpensive tomanufacture.

Reducing Resonance Effects

Prior art blower blade structures consisted of a tangential blower with23 longitudinal blades. These blades were mounted symmetrically at thecircumference of the structure. Substantial resonance effects weremeasured both with respect to fan parameters and actual laserperformance. Perturbations in the laser beam were shown to correspond toacoustic waves at 23 times the rotating frequency of the fan. Adverseaffects on bearing performance were also measured corresponding to 23times the fan's rotating frequency.

Improvements in fan structure design call for a non symmetrical bladearrangement such as that shown in FIG. 14A. An alternative as shown inFIG. 14B where the fan blade structure is formed of 16 separate machinedor cart segments with each segment having 23 blades is to rotate eachsegment by 360°/(15×23) or about 1° relative to the adjacent segment.Another improvement which is made relatively easy in the machine or castapproach to fan blade structure fabrication is to form the blades intoair foils as shown at 320 in FIG. 14C. Prior art blades were stamped anda cross section of the two of the stamped blades are shown forcomparison at 314. The direction of rotation is shown at 318 and 330represents the circumference of the blade structure. Whereasconventional blades are uniform in thickness, airfoil blades have a tearshape profile including a rounded leading edge, a thickened midsectionand a tapered trailing edge.

Bearing Improvements

Embodiments of the present invention will be made available with one oftwo alternative bearing improvements over the prior art.

Ceramic Bearings

A preferred embodiment of the present invention includes ceramicbearings. The preferred ceramic bearings are silicon nitride lubricatedwith a synthetic lubricant, preferably perfluoropolyalkylether (PFPE).These bearings provide substantially greater life as compared to priorart excimer laser fan bearings. In addition, neither the bearings northe lubricant are significantly affected by the highly reactive fluorinegas.

Magnetic Bearings

Another preferred embodiment of the present invention comes withmagnetic bearings supporting the fan structure as shown in FIG. 5. Inthis embodiment, the shaft 130 supporting the fan blade structure 146 isin turn supported by an active magnetic bearing system and driven by abrushless DC motor 130 in which the rotor 129 of the motor and therotors 128 of at least two bearings are sealed within the gasenvironment of the laser cavity and the motor stator 140 and the coils126 of the magnetic bearing magnets are located outside the gasenvironment. This preferred bearing design also includes an activemagnetic thrust bearing 124 which also has the coils located outside thegas environment.

Aerodynamic Anode Support Bar

As shown in FIG. 3, prior art gas flow from blower 10 was forced to flowbetween electrodes 6A and 6B by anode support bar 44. However,Applicants have discovered that the prior art designs of support bar 44such as that shown in FIG. 3 produced substantial aerodynamic reactionforces on the blower which were transferred to the blower bearingsresulting in chamber vibration. Applicants suspect that thesevibrational forces are responsible for blower bearing wear and possiblyoccasional bearing failures. Applicant has tested other designs, severalof which are shown in FIGS. 12A-12E, all of which reduced theaerodynamic reaction forces by distributing over a longer time period,the reaction force resulting each time a blade passes close to the edgeof support bar 44. One of Applicants preferred anode support bar designis shown in FIG. 6A at 84A. This design has substantially greater masswhich minimizes anode temperature savings. The total mass of the anodeand the anode support bar is about 3.4 Kg. Also, this design comprisesfins 84B which provides added cooling for the anode. Applicants testshave indicated that both of the acoustic baffles and the aerodynamicanode support bar tend to reduce slightly the gas flow so that is gasflow is limited, the utilization of these two improvements shouldinvolve a trade-off analysis. For these reasons two improvements areshown on FIG. 6A and not FIG. 6.

Pulse Power System Functional Description of Four Pulse Power Modules

A preferred pulse power system is manufactured in four separate modulesas indicated in FIGS. 8A and 8B, each of which becomes an important partof the excimer laser system and each of which can be quickly replaced inthe event of a parts failure or in the course of a regular preventativemaintenance program. These modules are designated by Applicants: highvoltage power supply module 20, commutator module 40, compression headmodule 60 and laser chamber module 80.

High Voltage Power Supply Module

High voltage power supply module 20 comprises a 300 volt rectifier 22for converting 208 volt three phase plant power from source 10 to 300volt DC. Inverter 24 converts the output of rectifier 22 to highfrequency 300 volt pulses in the range 100 kHz to 200 kHz. The frequencyand the on period of inverter 24 are controlled by the HV power supplycontrol board 21 in order to provide course regulation of the ultimateoutput pulse energy of the system. The output of inverter 24 is steppedup to about 1200 volts in step-up transformer 26. The output oftransformer 26 is converted to 1200 volts DC by rectifier 28 whichincludes a standard bridge rectifier circuit 30 and a filter capacitor32. DC electrical energy from circuit 30 charges 8.1 μF C₀ chargingcapacitor 42 in commutator module 40 as directed by HV power supplycontrol board 21 which controls the operation of inverter 24 as shown inFIG. 8A. Set points within HV power supply control board 21 are set bylaser system control board 100.

The reader should note that in this embodiment as shown in FIG. 8A thatpulse energy control for the laser system is provided by power supplymodule 20. The electrical circuits in commutator 40 and compression head60 merely serve to utilize the electrical energy stored on chargingcapacitor 42 by power supply module 20 to form at the rate of 2,000times per second an electrical pulse, to amplify the pulse voltage andto compress in time the duration of the pulse. As an example of thiscontrol, FIG. 8A indicates that processor 102 in control board 100 hascontrolled the power supply to provide precisely 700 volts to chargingcapacitor 42 which during the charging cycle is isolated from the downstream circuits by solid state switch 46. The electrical circuits incommutator 40 and compression head 60 will upon the closure of switch 46very quickly and automatically convert the electrical energy stored oncapacitor 42 into the precise electrical discharge pulse acrosselectrodes 83 and 84 needed to provide the next laser pulse as theprecise energy needed as determined by processor 102 in control board100.

Commutator Module

Commutator module 40 comprises C₀ charging capacitor 42, which in thisembodiment is a bank of capacitors connected in parallel to provide atotal capacitance of 8.1 μF. Voltage divider 44 provides a feedbackvoltage signal to the HV power supply control board 21 which is used bycontrol board 21 to limit the charging of capacitor 42 to the voltage(called the “control voltage”) which when formed into an electricalpulse and compressed and amplified in commutator 40 and compression head60 will produce the desired discharge voltage on peaking capacitor 82and across electrodes 83 and 84.

In this embodiment (designed to provide electrical pulses in the rangeof about 3 Joules and 16,000 volts at a pulse rate of 2000 Hz pulses persecond), about 250 microseconds (as indicated in FIG. 8F1) are requiredfor power supply 20 to charge the charging capacitor 42 to 800 volts.Therefore, charging capacitor 42 is fully charged and stable at thedesired voltage when a signal from commutator control board 41 closessolid state switch 44 which initiates the very fast step of convertingthe 3 Joules of electrical energy stored on charging capacitor C₀ into a16,000 volt discharge across electrodes 83 and 84. For this embodiment,solid state switch 46 is a IGBT switch, although other switchtechnologies such as SCRs, GTOs, MCTs, etc. could also be used. A 600 nHcharging inductor 48 is in series with solid state switch 46 totemporarily limit the current through switch 46 while it closes todischarge the C₀ charging capacitor 42.

Pulse Generation Stage

The first stage of high voltage pulse power productions is the pulsegeneration stage 50. To generate the pulse the charge on chargingcapacitor 42 is switched onto C₁ 8.5 μF capacitor 52 in about 5 μs asshown on FIG. 8F2 by closing IGBT switch 46.

First Stage of Compression

A saturable inductor 54 initially holds off the voltage stored oncapacitor 52 and then becomes saturated allowing the transfer of chargefrom capacitor 52 through 1:23 step up pulse transformer 56 to C_(p-1)capacitor 62 in a transfer time period of about 550 ns, as shown on FIG.8F3, for a first stage of compression 61.

The design of pulse transformer 56 is described below. The pulsetransformer is extremely efficient transforming a 700 volt 17,500 ampere550 ns pulse rate into a 16.100 volt, 760 ampere 550 ns pulse which isstored very temporarily on C_(p-1) capacitor bank 62 in compression headmodule 60.

Compression Head Module

Compression head module 60 further compresses the pulse.

Second Stage of Compression

An L_(p-1) saturable inductor 64 (with about 125 nH saturatedinductance) holds off the voltage on 16.5 nF C_(p-1) capacitor bank 62for approximately 550 ns then allows the charge on C_(p-1) to flow (inabout 100 ns) onto 16.5 nF Cp peaking capacitor 82 located on the top oflaser chamber 80 and which is electrically connected in parallel withelectrodes 83 and 84 and preionizer 56A. This transformation of a 550 nslong pulse into a 100 ns long pulse to charge Cp peaking capacitor 82makes up the second and last stage of compression as indicated at 65 onFIG. 8A.

Laser Chamber Module

About 100 ns after the charge begins flowing onto peaking capacitor 82mounted on top of and as a part of the laser chamber module 80, thevoltage on peaking capacitor 82 has reached about 14,000 volts anddischarge between the electrodes begins. The discharge lasts about 50 nsduring which time lasing occurs within the optical resonance chamber ofthe excimer laser. The optical resonance chamber is defined by a lineselection package 86 comprised in this example by a 2 prism wavelengthselector and a R-max mirror together, indicated as 86 in FIG. 8A and anoutput coupler 88. The laser pulse for this laser is a narrow band, 20to 50 ns, 157 nm pulse of about 10 ml and the repetition rate up to 2000pulses per second. The pulses define a laser beam 90 and the pulses ofthe beam are monitored by photodiode 92, all as shown in FIG. 8A.

Control of Pulse Energy

The signal from photodiode 92 is transmitted to processor 102 in controlboard 100 and the processor uses this energy signal and preferably otherhistorical pulse energy data (as discussed below in the section entitledPulse Energy Control Algorithm) to set the command voltage for the nextand/or future pulses. In a preferred embodiment in which the laseroperates in a series of short bursts (such as 100 pulse 0.5 secondbursts at 2000 Hz separated by a dead time of about 0.1 second)processor 102 in control board 100 is programmed with a specialalgorithm which uses the most recent pulse energy signal along with theenergy signal of all previous pulses in the burst along with otherhistorical pulse profile data to select a control voltage for thesubsequent pulse so as to minimize pulse-to-pulse energy variations andalso to minimize burst-to-burst energy variations. This calculation isperformed by processor 102 in control board 100 using this algorithmduring a period of about 35 μs. The laser pulses occurs about 5 μsfollowing the T₀ firing of IGBT switch 46 shown on FIG. 8F3 and bout 20μs are required to collect the laser pulse energy data. (The start ofthe firing of switch 46 is called T₀.) Thus, a new control voltage valueis thus ready (as shown on FIG. 8F1) about 70 microseconds after thefiring of IGBT switch 46 for the previous pulse (at 2,000 Hz the firingperiod is 500 μs). The features of the energy control algorithm aredescribed below and are described in greater detail in U.S. patentapplication Ser. No. 09/034,870 which is incorporated herein byreference.

Energy Recovery

This preferred embodiment is provided with electronic circuitry whichrecovers excess energy onto charging capacitor 42 from the previouspulse. This circuitry substantially reduces waste energy and virtuallyeliminates after ringing in the laser chamber 80.

The energy recovery circuit 57 is comprised of energy recovery inductor58 and energy recovery diode 59, connected in series across Co chargingcapacitor 42 as shown in FIG. 8B. Because the impedance of the pulsepower system is not exactly matched to that of the chamber and due tothe fact that the chamber impedance varies several orders of magnitudeduring the pulse discharge, a negative going “reflection” is generatedfrom the main pulse which propagates back from the chamber towards thefront end of the pulse generating system. After the excess energy haspropagated back through the compression head 60 and the commutator 40,switch 46 opens up due to the removal of the trigger signal by thecontroller. The energy recovery circuit 57 reverses the polarity of thereflection which has generated a negative voltage on the chargingcapacitor 42 through resonant free wheeling (a half cycle of ringing ofthe L-C circuit made up of the charging capacitor 42 and the energyrecovery inductor 58) as clamped against reversal of current in inductor58 by diode 59. The net result is that substantially all of thereflected energy from the chamber 80 is recovered from each pulse andstored on charging capacitor 42 as a positive charge ready to beutilized for the next pulse. FIGS. 8F1, 2 and 3 are time line chartsshowing the charges on capacitor Co, C₁, C_(p-1) and Cp. The charts showthe process of energy recovery on Co.

Magnetic Switch Biasing

In order to completely utilize the full B-H curve swing of the magneticmaterials used in the saturable inductors, a DC bias current is providedsuch that each inductor is reverse saturated at the time a pulse isinitiated by the closing of switch 46.

In the case of the commutator saturable inductors 48 and 54, this isaccomplished by providing a bias current flow of approximately 15 Abackwards (compared to the directional normal pulse current flow)through the inductors. This bias current is provided by bias currentsource 120 through isolation inductor LB1. Actual current flow travelsfrom the power supply through the ground connection of the commutator,through the primary winding of the pulse transformer, through saturableinductor 54, through saturable inductor 48, and through isolationinductor LB1 back to the bias current source 120 as indicated by arrowsB1.

In the case of compression head saturable inductor, a bias current B2 ofapproximate 5A is provided from the second bias current source 126through isolation inductor LB2. At the compression head, the currentsplits and the majority B2-1 goes through saturable inductor Lp-b 164and back through isolation inductor LB3 back to the second bias currentsource 126. A smaller fraction of the current B2-2 travels back throughthe HV cable connecting the compression head 60 and the commutator 40,through the pulse transformer secondary winding to ground, and through abiasing resistor back to the second bias current source 126. This secondsmaller current is used to bias the pulse transformer so that it is alsoreset for the pulsed operation. The mount of current which splits intoeach of the two legs is determined by the resistance in each path and isintentionally adjusted such that each path receives the correct amountof bias current.

Direction of Current Flow

In this embodiment, we refer to the flow of pulse energy through thesystem from a standard three-phase power source 10 to the electrodes andto ground beyond electrode 84 as “forward flow” and this direction asthe forward direction. When we refer to an electrical component such asa saturable inductor as being forward conducting we mean that it isbiased into saturation to conduct “pulse energy” in a direction towardthe electrodes. When it is reverse conducting it is biased intosaturation to conduct energy in a direction away from the electrodestoward the charging capacitor. The actual direction of current flow (orelectron flow) through the system depends on where you are within thesystem. The direction of current flow is now explained to eliminate thisas a possible source of confusion.

By reference to FIGS. 8A and 8B, in this preferred embodiment Cocapacitor 42 is charged to (for example) a positive 700 volts such thatwhen switch 46 is closed current flows from capacitor 42 throughinductor 48 in a direction toward C₁ capacitor 52 (which means thatelectrons are actually flowing in the reverse direction). Similarly, thecurrent flow is from C₁ capacitor 52 through the primary side of pulsetransformer 56 toward ground. Thus, the direction of current and pulseenergy is the same from charging capacitor 42 to pulse transformer 56.As explained below under the section entitled “Pulse Transformer”current flow in both the primary loops and the secondary loop of pulsetransformer 56 is toward ground. The result is that current flow betweenpulse transformer 56 and the electrodes during the initial portion ofthe discharge (which represents the main portion |typically about 80percent| of the discharge) is in the direction away from the electrodestoward transformer 56. Therefore, the direction of electron flow duringthe main discharge is from ground through the secondary of pulsetransformer 56 temporarily onto C_(p-1) capacitor 62 through inductor64, temporarily onto Cp capacitor 82, through inductor 81, throughelectrode 84 (which is referred to as the discharge cathode) through thedischarge plasma, through electrode 83 and back to ground. Thus, betweenpulse transformer 56 and the electrodes 84 and 83 during the maindischarge electrons flow in the same direction as the pulse energy.Immediately following the main portion of the discharge, currents andelectron flow are reversed and the reverse electron flow is from groundup through the grounded electrode 84, though the discharge space betweenthe electrodes to electrode 83 and back through the circuit throughtransformer 56 to ground. The passage of reverse electron flow throughtransformer 56 produces a current in the “primary” loops of transformer56 with electron flow from ground through the “primary” side of pulsetransformer 56 (the same direction as the current flow of the mainpulse) to ultimately charge Co negative as indicated qualitatively inFIG. 8F2. The negative charge on Co is reversed as shown in FIG. 8F2 andexplained above in the section entitled Energy Recovery.

Detailed Description of Pulse Power Components Power Supply

A more detailed circuit diagram of the power supply portion of thepreferred embodiment is shown in FIG. 8C. As indicated in FIG. 8C,rectifier 22 is a 6 pulse phase controlled rectifier with a plus 150v to−150V DC output. Inverter 24 is actually three invertors 24A, 24B and24C. Invertors 24B and 24C are turned off when the voltage on 8.1 μF Cocharging capacitor 42 is 50 volts less than the command voltage andinverter 24A is turned off when the voltage on Co 42 slightly exceedsthe command voltage. This procedure reduces the charge rate near the endof the charge. Step up transformers 26A, 26B and 26C are each rated at 7kw and transform the voltage to 1200 volt AC.

Three bridge rectifier circuits 30A, 30B and 30C are shown. The HV powersupply control board 21 converts a 12 bit digital command to an analogsignal and compares it with a feedback signal 45 from Co voltage monitor44. When the feedback voltage exceeds the command voltage, inverter 24Ais turned off as discussed above. Q2 switch 34 closes to dissipatestored energy within the supply. Q3 isolation switch 36 opens to preventany additional energy leaving the supply and Q1 bleed switch 38 closesto bleed down the voltage on Co 42 until the voltage on Co equals thecommand voltage. At that time Q1 opens.

Commutator and Compression Head

The principal components of commutator 40 and compression head 60 areshown on FIGS. 8A and 8B and are discussed above with regard to theoperation of the system. In this section, we describe details offabrication of the commutator.

Solid State Switch

In this preferred embodiment solid state switch 46 is an P/N CM 1000HA-28H IGBT switch provided by Powerex, Inc. with offices in Youngwood,Pa.

Inductors

Inductors 48, 54 and 64 comprise saturable inductors similar to thosedescribed in U.S. Pat. Nos. 5,448,580 and 5,315,611. A top and sectionview of a preferred saturable inductor design is shown respectively inFIGS. 8G1 and 8G2. In the inductors of this embodiment, flux excludingmetal pieces such as 301, 302, 303 and 304 are added as shown in FIG.8G2 in order to reduce the leakage flux in the inductors. The currentinput to this inductor is a screw connection at 305 to a bus alsoconnected to capacitor 62. The current makes four and one half loopsthrough vertical conductors. From location 305 the current travels downa large diameter conductor in the center labeled 1A, up six smallerconductors on the circumference labeled 1B, down 2A, up 2B, down all ofthe flux excluder elements, up 3B, down 3A, up 4B and down 4A, and thecurrent exits at location 306. Where a pot like housing 64A serves as ahigh voltage current lead. The “lid” 64B of the saturable inductor iscomprised of an electrical insulator material such as teflon. In priorart pulse power systems, oil leakage from oil insulated electricalcomponents has been a problem. In this preferred embodiment, oilinsulated components are limited to the saturable inductors and the oilis contained in the pot-like oil containing metal housing 64A which is,as stated above, the high voltage connection output lead. All sealconnections are located above the oil level to substantially eliminatethe possibility of oil leakage. For example, the lowest seal in inductor64 is shown at 308 in FIG. 8G2. Since the flux excluding metalcomponents are in the middle of the current path through the inductor,the voltage allowing a reduction in the safe hold-off spacing betweenthe flux exclusion metal parts and the metal rods of the other turns.Fins 307 are provided to increase heat removal.

Capacitors

Capacitor banks 42, 52 and 62 are all comprised of banks of commerciallyavailable off-the-shelf capacitors connected in parallel. Thesecapacitors are available from suppliers such as Murata with offices inSmyrna, Ga. Applicants preferred method of connecting the capacitors andinductors is to solder or bolt them to positive and negative terminalson special printed circuit board having heavy nickel coated copper leadsin a manner similar to that described in U.S. Pat. No. 5,448,580.

Pulse Transformer

Pulse transformer 56 is also similar to the pulse transformer describedin U.S. Pat. Nos. 5,448,580 and 5,313,481; however, the pulsetransformers of the present embodiment has only a single turn in thesecondary winding and 23 separate primary windings. A drawing of pulsetransformer 56 is shown in FIG. 8D. Each of the 23 primary windingscomprise an aluminum spool 56A having two flanges (each with a flat edgewith threaded bolt holes) which are bolted to positive and negativeterminals on printed circuit board 56B as shown along the bottom edge ofFIG. 8D. Insulators 56C separates the positive terminal of each spoolfrom the negative terminal of the adjacent spool. Between the flanges ofthe spool is a hollow cylinder 1{fraction (1/16)} long with a 0.875 ODwith a wall thickness of about {fraction (1/32)} inch. The spool iswrapped with one inch wide, 0.7 mil thick Metglas™ 2605 S3A and a 0.1mil thick myler film until the OD of the insulated Metglas™ wrapping is2.24 inches. A prospective view of a single wrapped spool forming oneprimary winding is shown in FIG. 8E.

The secondary of the transformer is a single stainless steel rod mountedwithin a tight fitting insulating tube of electrical glass. The windingis in four sections as shown in FIG. 8D. The stainless steel secondaryshown as 56D in FIG. 8D is grounded to a ground lead on printed circuitboard 56B at 56E and the high voltage terminal is shown at 56F. Asindicated above, a 700 volt pulse between the + and − terminals of theprimary windings will produce a minus 16.100 volt pulse at terminal 56Fon the secondary side for a 1 to 23 voltage transformation. This designprovides very low leakage inductance permitting extremely fast outputrise time.

Laser Chamber Pulse Power Components

The Cp capacitor 82 is comprised of a bank of twenty-eight 0.59 nfcapacitors mounted on top of the laser chamber pressure vessel. Theelectrodes 83 and 84 are each solid brass bars about 28 inches longwhich are separated by about 0.5 to 1.0 inch. In this embodiment, thetop electrode 83 is the cathode and the bottom electrode 84 is connectedto ground as indicated in FIG. 8A.

Compression Head Mounting

This preferred embodiment of the present invention includes acompression head mounting technique shown in FIGS. 8H1 and 8H2. FIG. 8H1is a side section view of the laser system showing the location of thecompressor head module 60 in relation to electrodes 83 and 84. Thistechnique was designed to minimize the impedance associated with thecompression lead chamber connection and at the same time facilitatesquick replacement of the compression head. As shown in FIGS. 8H1 and 8H2the ground connection is made with an approximately 28 inch long slottab connection along the back side of the compression head as shown at81A in FIG. 8H1 and 81B in FIG. 8H2. The bottom of the slot tab isfitted with flexible finger stock 81C. A preferred finger stock materialis sold under the trade name Multilam®.

The high voltage connection is made between a six-inch diameter smoothbottom of saturable inductor 64 and a mating array of flexible fingerstock at 89 in FIG. 8H1. As above, a preferred finger stock material isMultilam®. This arrangement permits the replacement of the compressionhead module for repair or preventative maintenance in about fiveminutes.

Gas Control Module

This preferred embodiment comprises a fluorine control system whichpermits operation within a chosen sweet spot without the use of afluorine monitor. This embodiment can be described by reference to FIG.16.

Fluorine Depletion

Laser chamber 1 comprises about 20.3 liters of laser gas. Nominally asdescribed above, the constituents are 0.1 percent fluorine, and theremainder helium at a pressure of about 4 atmospheres. The 0.1 percentfluorine represents a volume of about 0.0023 liters or 2.3 ml offluorine at 4 atm. In mass terms the nominal amount of fluorine in thelaser chamber is about 110 mg. The partial pressure of the pure fluorineis about 411 Pa, pure fluorine (corresponding to about 41 kPa of the 1%fluorine mixture). During normal operations with the laser operating ata duty factor of about 40 percent (which is typical for a lithographylaser) fluorine is depleted at a rate of about 4.5 mg per hour (thiscorresponds to about 4% of the fluorine in the chamber per hour). Interms of partial pressure of pure fluorine, this normal depletion rateof fluorine is about 16 Pa per hour. To make up for this depletion usingthe 1% fluorine gas mixture, a volume of the mixture equivalent to about1.6 kPa per hour is added to the chamber.

The fluorine depletion rate for the laser is far from constant. If thelaser fan is operating but no lasing is taking place the fluorinedepletion rate is cut approximately in half. If the fan is shutdown thefluorine depletion rate is cut to about ¼ the 40% duty factor depletionrate. At 100% duty factor the depletion rate is about double the 40%duty factor depletion rate.

Gas Replacement

The process described above basically replaces depleted fluorine on analmost continuous basis. Since the fluorine gas source is only 1%fluorine it also replaces a portion of the He in the chamber on analmost continuous basis. Nevertheless, even through a portion of thelaser gas is being substantially continuously replaced, operation inthis mode results in a build up of contaminants in the laser gas whichreduces the efficiency of the laser. This reduction in efficiencyrequires an increase in the voltage and/or an increase in the fluorineconcentration to maintain the desired pulse energy. For this reason,normal practice with prior art systems suggest that periodically thelaser be shutdown for a substantially complete gas exchange. Thissubstantially complete gas exchange is referred to as a refill. Theseperiods may be determined based on number of laser pulses such as100,000,000 pulses between refills, or refill times may be determinedbased on calendar time since the last refill or a combination of pulsesand calendar time. Also the refill times may be determined by themagnitude of the charging voltage needed for a desired output at aparticular fluorine concentration. Preferably after a refill a new testfor the “sweet spot” should be run. Also, periodically in between fillsthe sweet spot test should be performed so that if the sweet spotchanges the operator will know where the new sweet spot is.

A refill may be accomplished using the system shown in FIG. 16 asfollows. With valves 510, 506, 515, 512, 517, and 504 closed, valves 506and 512 are opened, vacuum pump 513 is operated and the laser chamber ispumped down to an absolute pressure of less than 13 kPa. (A direct pumpdown line may be provided between the chamber 1 and vacuum pump 513 topermit a quick pump down.) Valve 512 is closed. Valve 516 is opened and100% He buffer gas from buffer gas bottle 516 is added to the chamber tofill it to a pressure equivalent to 262 kPa at 50° C. (Note that forthis 20.3 liter laser chamber, temperature correction can beapproximated using a ΔP/ΔT correction of 1 kPa/° C. for a chambertemperature deviation from 50° C. So if the chamber temperature is 23°C. it would be filled to 247 kPa.) Valve 517 is closed and valve 515 isopened and a quantity of the 1% Fl. 99% He mixture from halogen rich gasbottle 514 is added to chamber 1 to fill it to a pressure equivalent to290 kPa at 50° C. (Note a temperature correction should be used.) Thiswill produce a gas mixture in the chamber of approximately 0.1% Fl and99% He. When the chamber is heated to about 50° C. the pressure will beabout 4 atm.

N₂ Purge System

Because O₂ strongly absorbs 157 mn light, O₂ must be excluded from thebeam path. Applicants have developed an N₂ purge system that is greatlyimproved over prior art systems. All optical components associated withthe laser that are outside the chamber are purged with nitrogen. Thisnitrogen system is operated at a pressure that is during operation ofthe laser only about 10 pascals in excess of atmospheric pressure. Thissmall pressure differential is preferred to avoid a pressure distortioneffect on the optical components. Components purged include the linenarrowing module, the output coupler, the wavemeter and the shutterassembly.

Seals are provided at all potential leakage sites output portsconnecting of {fraction (1/16)}-inch id. tubes about 6 feet long areprovided. The flow through the output ports is monitored to assureproper functioning of the purge system. Preferred flow rates of about41/minute through the {fraction (1/16)}-inch id. 6-foot long tube ispreferred flow rate to correspond to the desired N₂ pressuredifferential.

Laser Component Cooling

Preferred embodiments of the present invention which is especiallyuseful for operation at repetition rates in excess of 1000 to 2000 Hz,includes a unique cooling technique shown in FIG. 13 for cooling anexcimer laser.

Components of the laser are contained in enclosure 240 which ismaintained on the inside at a slight vacuum produced by a blower mountedin a vent as shown at 224 in FIGS. 13 and 4A. The cabinet comprisesfiltered intake port 241 near the top of the cabinet and a few smallleakage sources, such as around gasketed doors, so that the flow of roomair through the laser enclosure is about 200 ft³/min which is not nearlysufficient to remove the heat produced by heat producing components ofthe laser.

The very great majority (roughly 90 percent) of the waste heat producedby the laser (roughly 12 kw at 100% duty factor) is removed by a chilledwater system as shown in FIG. 13.

In this embodiment the major heat sources in the laser are the highvoltage supply 20, the commutator 40, the compression head 60 and thelaser chamber 80. For the chamber a water cooled heat exchanger islocated inside the chamber and heat is transferred from circulatinglaser gas to the heat exchanger to the cooling water. Another heatexchanger (not shown) is mounted on an outside surface of the chamber.For the rest of the major heat producing components cooling water ispiped to the location of the component and one or more fans force airthrough a water-to-air heat exchanger onto the component as shown inFIG. 13. For the compression head the circulation is contained as shown,but for the HVPS and the commutator the circulation is onto thecomponent then through other portions of the enclosure to also coolother components before being recirculated back to the heat exchangers.

Dividing pans 242 and 243 guide the general ventilation air from filter241 through a path shown by open headed arrows 244 to vent 224.

This cooling system contains no ducts and except for a water linefeeding the heat exchangers inside of and attached to the laser chamberthere is no water line connection to any laser component. Since allcomponents (other than the laser chamber) are cooled by air blown aboutinside the enclosure, there are no cooling connections to make a breakwhen installing and replacing components. Also, the lack of need forducting greatly increases useable components and working space insidethe enclosure.

Pulse Energy Control Algorithm Mode of Operation—Chip Lithography

The embodiment of the present invention includes a computer controllerprogram with a new algorithm, which substantially reduces prior artvariations in pulse energy and total integrated burst energy. Theimproved equipment and software and a preferred process for reducingenergy sigma and burst dose variation is described below.

As stated in the background section of this specification, the burstmode is a typical mode of operation of an excimer laser used for thelight source of a stepper machine in the lithographic productionintegrated circuits. In this mode the laser is operated to produce “aburst” of pulse at the rate of 1000 Hz for about 110 milliseconds toproduce 110 pulses to illuminate a section of a wafer. After the burstthe stepper moves the wafer and the mask and once the move is completewhich takes typically a fraction of a second the laser produces another110 pulse burst. Thus, normal operation is bursts of about 110milliseconds followed by dead times of a fraction of a second. Atvarious times, longer dead time periods will be provided to that otheroperations can be performed. This basic process continues 24 hours aday, 7 days per week, for several months with the laser typicallyproducing several millions of bursts per day. In the above burst mode,it is usually important that each section of the wafer received the sameillumination emerge on each burst. Also, chip makers want the pulse topulse variation to be minimized.

This preferred embodiment of the present invention accomplishes theseobjectives with equipment and software which monitors the energy of eachpulse (pulse N-1) then controls the energy of the next pulse (pulse N)based on the results of a:

1) a comparison of the measured energy of pulse N-1 with a target pulseenergy and

(2) a comparison of the accumulated dose of the burst through pulse N-1to a target pulse dose through pulse N-1.

In the typical F₂ excimer laser we have been discussing the energy ofthe first 30-40 ms of a burst in typically less stable than the rest ofthe burst due to transient effects in the laser gas. After about 40 msfollowing the first pulse, the pulse energy at constant voltage isrelatively constant. In dealing with these early perturbations.Applicants have separated the burst into two time-wise regions, thefirst region (consisting of a number of the earlier pulses, for example,40 pulses) called the “K” region and a second region (consisting of thepulses which follow the K region) which Applicants, in thisspecification, refer to as the “L” region.

This embodiment of the present invention utilizes prior art excimerlaser equipment for pulse energy control. Pulse energy of each pulse ofeach burst is measured by photodiode 92 as shown in FIG. 8A. The overallresponse time of this photodiode and its sample and hold circuit,including time required to reset the circuit, is less than substantiallyless than 500 microseconds. The accumulated signal resulting from eachapproximately 15 ns pulse is stored a few microseconds after the pulseis over and this signal is read six times and the average is stored bycomputer controller 22 approximately 1.0 microsecond after the beginningof the pulse. The accumulated energy of all the previous individualpulses in a burst is referred to as the burst dose value. Computercontroller utilizes the signal representing the pulse energy of pulse Nalong with target pulse energy and the burst dose value in order tospecify the high voltage for the pulse N+1. This calculation requiresabout 200 microseconds. When the value of high voltage for N+1 isdetermined, computer controller sends a signal to the high voltagecommand (VCMD) of the high voltage power supply as shown in FIG. 8Aestablishing the charging voltage for pulse N+1 that takes a fewmicroseconds. Computer controller commands the high voltage power supplyto charge up capacitor Co to the specified voltage. (At high repetitionrates in excess of 2000 Hz it may be desirable to start the chargingbefore the calculation is complete.) The charging requires about 250microseconds so that Co is fully charged and ready to go when itreceives a trigger signal for pulse N+1 from trigger circuit 13 as shownin FIG. 2 at 0.5 millisecond after the trigger signal from pulse N. Onthe trigger signal, capacitor C₀ discharges its approximately 700 voltsinto the magnetic compression circuit shown in FIG. 8B over a period ofabout 5 microseconds and the pulse is compressed and amplified by themagnetic compression circuit to produce a discharge voltage on capacitorCp of about 16,100 volts which discharges across electrodes 6 in about100 ns producing a laser pulse of about 10 mJ and about 15 ns induration.

Preferred Algorithm

A special preferred process for adjusting the charging voltage toachieve substantially desired pulse energies when operating in a burstmode is described below.

The process utilizes two voltage adjustment algorithms. The firstalgorithm applies to the first 80 pulses and is called the KPIalgorithm. The second algorithm called the PI algorithm applies topulses after pulse number 40. This time period after the 80^(th) pulseis herein called the “L region” of the burst. The initials “PI” refer to“proportional integral” and the “K” in “KPI” refers to the “K region” ofthe burst.

KPI Algorithm

The K region comprises pulses 1 through k, where k=40 for this preferredembodiment. The algorithm for setting the charging voltage for pulse Nis:

V_(N)=(V_(B))_(N)−(V_(C))_(N−1)N=1,2, . . . k

where:

V_(N)=charging voltage for N'th pulse

(V_(B))_(N)=an array of k stored voltages which represent the currentbest estimate of the voltage required to produce the target energy E_(T)for the N'th pulse in the K region. This array is updated after eachburst according to the equation below.

(V_(C))_(N−1)=a voltage correction based the energy error of theprevious pulse and on the energy errors which occurred for the previouspulses in the burst, up to pulse N-1$\sum\limits_{i = 1}^{N - 1}\quad \frac{\left( {{A \cdot r_{1}} + {B \cdot D_{t}}} \right.}{\left( {}_{t}{{lE}/_{t}{lV}} \right)}$

By definition, (V_(c))_(o)=0

A,B=fractions typically between 0 and 1, which in this preferredembodiment both A and B are 0.5

ε_(i)=the energy error of the i'th pulse =E_(i)−E_(T), where E_(i) isthe energy for the i'th pulse, and E_(T) is the target energy

D_(i)=the cumulative dose error of the burst, including all pulses from1 through i $\sum\limits_{k = 1}^{i}\quad \varepsilon_{k}$

dE/dV=a rate of change of pulse energy with charging voltage. (In thisembodiment, one or more values of dE/dV is determined experimentallyduring each burst and a running average of these values is used for thecalculation)

The stored values (VB)N are updated during or after each burst accordingto the following relation:${\left( V_{B} \right)_{N}^{M + 1} = {\left( V_{B} \right)_{N}^{M} - {C \cdot \left( {\frac{\varepsilon_{N}}{{E}/{V}} - \left( V_{C} \right)_{N}} \right)}}},$

where the index M refers to the burst number

C=a fraction typically between 0 and 1, which in this preferredembodiment is 0.3.

PI Algorithm

The L region comprises pulses k+1 to the end of the burst (for apreferred embodiment, pulse numbers 41 and higher). The algorithm forsetting the charging voltage for pulse N is:${V_{N} = {{V_{N - 1} - {\frac{\left( {{A \cdot \varepsilon_{N - 1}} + {B \cdot D_{N - t}}} \right)}{\left( {{E}/{V}} \right)}N}} = {k + 1}}},{K + 2},\ldots$

where:

V_(N)=charging voltage for N'th pulse

V_(N−1)=charging voltage for N−1'st (previous) pulse

The variables A, B, ε_(i), D₁, and dE/dV are defined as before.

Determination of dE/Dv

A new value for dE/dV is determined periodically, in order to track therelatively slow changes in the characteristics of the laser. In thepreferred embodiments, dE/dV is measured by varying or dithering thevoltage in a controlled manner during two successive pulses in the Lregion. For these two pulses, the normal PI energy control algorithm istemporarily suspended and replaced by the following:

For pulse j:$V_{j} = {V_{j - 1} - \frac{\left( {{A \cdot \varepsilon_{j - 1}} + {B \cdot D_{j - 1}}} \right)}{\left( {{E}/{V}} \right)} + V_{Dither}}$

where V_(Dither)=a fixed voltage increment, typically a few volts

For pulse j+1:

V_(j+1)=V_(j)−2.V_(Dither)

After pulse j+1, dE/dV is calculated:${{E}/{V}} = \frac{\left( {E_{j + 1} - E_{j}} \right)}{2 \cdot V_{Dither}}$

The calculation of dE/dV can be very noisy, since the expected energychanges due to the dithering voltage can be of the same magnitude as thenormal energy variation of the laser. In the preferred embodiment, arunning average of the last 50 dE/dV calculations is actually used inthe PI and KPI algorithms.

The preferred method for V_(Dither) choosing is to specify a desiredenergy dither E_(Dither), typically a few percent of the energy targetE_(T), and then use the current (averaged) value for dE/dV to calculateV_(Dither): $V_{Dither} = \frac{E_{Dither}}{\left( {{E}/{V}} \right)}$

Pulse j+2 (immediately following the two dithered pulses) is notdithered, but has the special value:$V_{j - 2} = {V_{j - 1} + V_{Dither} - {\frac{\left( {{A \cdot \left( {\varepsilon_{j + 1} + E_{Dither}} \right)} + {B \cdot D_{j + 1}}} \right)}{\left( {{E}/{V}} \right)}\left( {{{pulse}{\quad \quad}j} + 2} \right)}}$

This special value for V_(j+2) is corrected for both the applied voltagedither and the expected energy dither from pulse j+1.

Many variations on the algorithm described above are possible. Forexample, dE/dV can be determined in the L region as well as the K. Thedithering can be performed once per burst, or several times. Thedithering sequence may be performed at a fixed pulse number j asdescribed above, or it may be initiated for a randomly chosen pulsenumber which varies from one burst to the next.

The reader should recognize that A, B and C are convergence factors,which could have many other values. Higher values than those specifiedabove could provide quicker convergence but could lead to increasedinstability. In another preferred embodiment, A={square root over (2B)}. This relationship is developed from a recognized technique to producecritical damping. B could be zero in which case there would be no dosecorrection; however, A should not be zero because it provides adampening term for the dose conveyance portions of the algorithm.

If the determined value of dE/dV becomes too small the above algorithmcould cause over correction. Therefore a preferred technique is toarbitrarily double dE/dV if the energy sigma value exceeds a threshold.Default values of V and dE/dV are provided for the first pulse of aburst. D is set to zero at the start of each burst. The default dE/dV isset at about three times the expected dE/dV to avoid initial overcorrection.

An alternate method for determining dE/dV without the dither referred toabove is to merely measure and store the energy and voltage valuesduring laser operation. (Measured rather than specified voltage valuescan also be used.) These data can be used to determine dE/dV as afunction of V for constant pulse energy. The reader should note thateach individual value of dE/dV would contain fairly large uncertaintiesbecause the elements of the value are differences of measurements havingsignificant uncertainties. However, averaging large number of dE/dVvalues can reduce these uncertainties. The dither exercise to determinedE/dV does not have to be made on each burst but instead could be doneperiodically such as once every M bursts. Or the measurement of dE/dVcould be replaced by a calculation performed by the computer or thevalue of dE/dV could be inserted manually by the operator of theprevious pulse for the calculation of V_(N+1). An alternate approachwould be to use the actual measured value for V_(N) for this controlsystem. Also the value of V_(BIN) are calculated from specified values,not actual measure values in the above-described embodiment. An obviousalternative would be to use measured voltage values. E_(T) is normally aconstant value such as 10 mJ but it does not have to be constant. Forexample, E_(T) of the last ten pulses could be smaller than the nominalpulse energy so that percentage deviations from target E_(T) for thesepulses would have a smaller effect on the integrated pulse dose. Also,it may be preferable in some situations to program computer controller22 to provide E_(T) values that vary from burst to burst.

Single Line and Narrow Line Configurations

FIG. 11A shows a preferred single line configuration for a preferred F₂laser system. In this configuration one of two major F2 lines isselected with a simple prism selector as shown in the figure. FIG. 11Bshows a preferred line narrowed system in which a power oscillator isseeded by a master oscillator.

Prototype Unit

A prototype F₂ laser system unit was built and tested by Applicants andtheir fellow workers.

The prototype laser is largely based on current production KrF and ArFlasers incorporating several important improvements over prior artexcimer laser systems, utilizing a high efficiency chamber andsolid-state pulsed power excitation. The discharge is corona pre-ionizedto minimize gas contamination. The entire optical beam path is nitrogenpurged to avoid light absorption by oxygen and to avoid damage tooptical components. All resonator optics were external to the angledchamber window equipped laser chamber. The gas mixture was 0.1% fluorinein 4 atmospheres of helium and the electrode gap was reduced to 10 mm.

With this unit, Applicants have successfully demonstrated the keyparameters for a prototype lithography fluorine laser with only fewstraight-forward changes in an ArF laser currently in development.

Experimental Results

Experimental results from experiments with the prototype unit aredescribed below. The laser power was measured by a standard power meterand cross-correlated with a piezo-electric Joulemeter. Contributions ofthe red, atomic fluorine laser were subtracted and usually amounted toless than 1% of the total energy. By venting the beam delivery tubes toair, which strongly absorbs 157 nm light, the red radiation could bemeasured.

The laser wavelength in this prototype unit was operated in single-linemode at 157.6 nm by tuning with a set of two external prisms. The lasercould also be tuned to the 157.5 nm transition line with reducedefficiency. The transition at 156.7 run was not observed. The laserspectrum as recorded by a 2 meter Jobin Yvon VUV spectrometer indicatesa measurement-limited linewidth of 6 pm.

In broadband (or multi-line) operation a maximum power of 12 W wasobtained at a repetition rate of 1000 Hz. The power increased linearlywith repetition rate without signs of saturation. The behavior in singleline mode was similar, but at one third of the energy. This energydecrease is due to a large cavity length increase in the present prismset-up and can be significantly reduced. In burst mode a 3 sigmastability of 5% was recorded. Only a benign burst transient on theoutput energy was observed. This compares favorably to ArF lasers whichshow larger energy instability and a gas flow related burst transient.From this one can conclude that a production fluorine lithography laserwill have even better energy stability than present day ArF lasers. Theintegral square pulse duration was 30 ns which is approaching theperformance of ArF lasers.

Careful selection of laser chamber materials and preionization by acorona discharge enabled laser operation without cryogenic purificationand without halogen injection over a period of several hours and 3 Mshots with minimal energy degradation.

The dependence of broadband laser power upon repetition rate isdisplayed in the top portion of FIG. 10A. The bottom portion of FIG. 10Ashows a slight drop off of pulse energy with increasing repetition rateabove 1000 Hz. The laser power increases almost linearly with repetitionrate up to a power of 15 W at 1 kHz and about 19 W at 2000 Hz. Based onthis linear relationship one can assume that the fluorine laser can bescaled further to several kilohertz operation, providing the gas flow isscaled accordingly. Since Applicants are using helium as a buffer gas,only a fraction of the blower power of standard neon based lasers isrequired and therefore does not present a limitation to higher flowspeeds.

A good measure for energy stability is gained by observing the energytransient in burst mode. For this the laser is repeatedly fired inbursts and the average energy for every pulse position in the burst isrecorded. Also, for every pulse number in the burst the averagevariation in energy from burst to burst is calculated. The resultingenergy and stability curves for the fluorine laser and for comparisonalso for a line-narrowed ArF laser are displayed in FIGS. 10B1 and 2.The fluorine laser exhibits only minor energy variations over a 120 shotburst. The energy stability shows an initial increase in the beginningof the bursts and then stabilizes on a 3 sigma level of about 3%. Bycontrast the ArF laser exhibits a large transient in the energy and a 3sigma instability around 7%. The ArF laser obtained a dose stability of0.5% in a 60 pulse window, therefore the fluorine laser is expected todeliver at least the same dose stability. FIG. 10C comprises pulseenergy and 3σ values at 1000 Hz and 1900 Hz.

A spectrum of the broadband fluorine laser as recorded by VUVspectrometer is shown in FIGS. 10D1 and 2. Clearly visible are the twotransition lines at 157.52 nm and at 157.63 nm, 87% of the laser energyis located in the longer wavelength line at 157.63 nm. The transition at156.7 nm was not observed. Single-line mode operation at 157.63 nm wasachieved by tuning with a set of two external prisms. The laser couldalso be tuned to the 157.52 nm transition line, but at reducedefficiency. Also shown in FIGS. 10D1 and 2 is an expanded view of thelaser line at 157.63 nm. Convolved linewidths of 1.14 pm FWHM and 2.35pm 95% were measured. These linewidths are much narrower than previouslyexpected. Therefore, a line selected fluorine laser without additionalline-narrowing will be sufficient for all but fully refractive imaginesystems. The laser power vs. repetition rate behavior of the single linelaser exhibits the same linear rise as the broadband laser. However, themaximum power in this initial experiment was limited to 4 W. The reducedoutput power was caused by reflection losses in the line selectionoptics and by an overly long cavity length.

The horizontal and vertical beam profiles were measured at 1 m distancefrom the laser. (See FIGS. 10E and 2). The beam shows smooth profileswith a high degree of symmetry. These kinds of profiles are easilymanaged by currently used homogenizer technology to produce very uniformillumination.

As estimation of the gas lifetime is derived by operating the fluorinelaser at constant voltage without fluorine injection and recording theevolution of laser power versus the number of shots. No cryogenicpurification was used in these measurements. As evident in FIG. 10F, thelaser power decreases by less than 20% after 4 million laser shots,which is at least as good as for comparable ArF lasers. From previousexperience with ArF lasers one can thus estimate a gas lifetime of about25 million shots by making use of periodic fluorine injections. This isclearly a result of the choice of compatible materials in the laserchamber and the use of corona pre-ionization. For KrF and ArF lasers adirect correlation between fluorine consumption and chamber lifetime hasbeen established, previously. We can therefore estimate a chamberlifetime of the fluorine laser on the same order as that of an ArFlaser.

FIG. 15A bottom graph shows pulse energy versus F₂ concentration and thetop graph shows the maximum pulse repetition rate, both at a blowerspeed of 2500 rpm. FIG. 15B top graph shows pulse shape as function oftime indicating a FWHM of 16 ns, and the bottom graph shows that theintegrated square pulse width is about 37 ns.

Although this F₂ laser system has been described with reference to aparticular embodiment, it is to be appreciated that various adaptationsand modifications may be made. For example, many alternative embodimentsare discussed in the patent applications listed in the first sentence ofthis specification, all of which have been incorporated herein byreference. An etalon output coupler could be used to provide additionalline narrowing. The buffer gas could be neon instead of helium. Theinvention is to be limited only by the appended claims.

What is claimed is:
 1. A very narrow band reliable modular productionquality high repetition rate ArF F₂ excimer laser for producing a narrowband pulsed laser beam at repetition rates of at least about 1000 Hz,said laser comprising: A. a quickly replaceable laser chamber modulecomprising a laser chamber comprising: 1) two elongated electrodes; 2) alaser gas comprised of a) fluorine, and b) a noble gas; 3) a gascirculator for circulating said gas between said electrodes at speeds ofat least two cm/millisecond B. a modular pulse power system comprised ofat least one quickly replaceable module, said system being comprised ofa power supply and pulse compression and amplification circuits andpulse power controls for producing high voltage electrical pulses of atleast 14,000 volts across said electrodes at rates of at least about1000 Hz; and C. a laser pulse energy control system for controlling thevoltage provided by said pulse power system, said control systemcomprising a laser pulse energy monitor and a computer processorprogrammed with an algorithm for calculating, based on historical pulseenergy data, electrical pulses needed to produce laser pulses havingpulse energies within a desired range of energies.
 2. A laser as inclaim 1 wherein said chamber, said pulse power system, said linenarrowing system, said energy control system and substantially allelectrical, optical and mechanical components of said laser arecontained in quickly replaceable modules.
 3. A laser as in claim 2wherein said two elongated electrodes define a cathode and an anode andsaid anode is supported by an anode support bar having a tapered surfacepositioned to reduce aerodynamic reaction forces on said bearings.
 4. Alaser as in claim 1 wherein said chamber and said gas circulator definea gas flow path and an upstream direction and said laser also comprisesa single preionizer tube located upstream of said electrodes.
 5. A laseras in claim 1 wherein each of said electrodes define an electrode lengthand said single preionizer tube is comprised of a grounded electricallyconducting rod positioned along the axis of an Al₂O₃ hollow cylindricaltube having a length longer than said electrode length.
 6. A laser as inclaim 1 wherein all of said at least three prisms are comprised ofcalcium fluoride.
 7. A laser as in claim 1 wherein said two elongatedelectrodes define a cathode and an anode and said anode support barcomprises cooling fins.
 8. A laser as in claim 7 herein said anode andsaid anode support bar together have a combined mass of at least about3.4 kg.
 9. A laser as in claim 1 wherein said two elongated electrodesdefine a cathode and an anode and said anode comprises cooling fins. 10.A laser as in claim 1 wherein said modular pulse power system iscomprised of at least three modules which each of which are designed forquick removal and replacement.
 11. A laser as in claim 1 wherein saidlaser chamber defines a chamber structure and wherein said two elongatedelectrodes define a cathode and an anode and said cathode is insulatedfrom said chamber structure by a single piece insulator comprised ofAl₂O₃ which is attached to a portion of said chamber structure.
 12. Alaser as in claim 11 wherein the portion of said chamber structure towhich said single piece insulator is comprised of a material having acoefficient of thermal expansion similar to that of Al₂O₃.
 13. A laseras in claim 12 wherein said structure material is ASTM A36 steel.
 14. Alaser as in claim 13 wherein said cathode is mounted directly on saidsingle piece insulator.
 15. A laser as in claim 1 wherein all sealsexposed to said laser gas are metal seals.
 16. A laser as in claim 1further comprising flow vane structures comprised of monel.
 17. A laseras in claim 1 and further comprising acoustic baffles.
 18. A laser as inclaim 1 wherein said power supply comprises a rectifier for convertingAC power to DC power, an inverter for converting the DC power to highfrequency AC power, a step-up transformer for increasing the voltage ofsaid high frequency AC power to a higher voltage, a rectifier forconverting the higher voltage to charge a charging capacitor to avoltage at or approximately at a command voltage established by saidlaser pulse energy control system.
 19. A laser as in claim 18 whereinsaid power supply is configured to slightly over charge said chargingcapacitor and further comprises a bleed circuit to bleed down saidcharging capacitor to said command voltage.
 20. A laser as in claim 19wherein said pulse power system comprises a solid state switch whichupon closing initiates said high voltage electrical pulses by allowingcharge to flow from said charging capacitor to a second capacitor tocreate a high voltage charge on said record capacitor.
 21. A laser as inclaim 20 and further comprising an inductor, a pulse transformer and athird capacitor wherein said inductor, pulse transformer and said thirdcapacitor are arranged to permit the high voltage charge on said secondcapacitor to flow to ground through the primary side of said pulsetransformer in order to produce a very high voltage pulse at the outputof said pulse transformer to be stored temporarily on said thirdcapacitor.
 22. A laser as in claim 21 wherein said primary side of saidpulse transformer comprises a plurality of hollow spools, each spooldefining an axis, connected in series and a secondary side of said pulsetransformer is comprised of at least one rod co-aligned with the axis ofa plurality of said spools.
 23. A laser as in claim 22 wherein said atleast one rod is four rods connected in series and defining two leads,one defining a ground lead and the other a very high voltage lead.
 24. Alaser as in claim 1 wherein said laser pulse power system comprises atleast one saturable inductor with a coil emersed in oil contained in apot which also serves as the high voltage lead of the inductor.
 25. Alaser as in claim 1 wherein said gas circulator comprises a blowercomprising a shaft supported by active at least two magnetic bearings,each bearing comprising a stator and a rotor; said shaft bearing driverby a motor comprising a stator and a rotor, said blower also comprisinga sealing means for sealing said rotors within an environment containingsaid laser gas with said stator outside said laser gas environment. 26.A laser as in claim 1 wherein said gas circulator comprises a blowercomprising a shaft supported by at least two ceramic bearings.
 27. Alaser as in claim 1 and further comprising an N₂ purge system providingan N₂ purge flow to all laser optical components outside the laserchamber wherein said purge flow is contained at a pressure of less than10 pascals.
 28. A laser as in claim 1 and further comprising a means ofmeasuring the rate of change of pulse energy with changing voltageΔE/ΔV, and a computer controller programmed with an algorithm forcontrolling pulse energy and integrated energy dose in a burst of pulsesdefining present burst pulses, P₁, P₂ . . . P_(N) . . . P_(k). P_(k+1),P_(k+2) . . . P_(K+N) . . . P₁. P₂ . . . P_(N−1), P_(N), from said laserhaving a pulse power system including a high voltage charging systemdefining a charging voltage, said algorithm comprising the steps of: A)measuring the energy in each pulse of said burst of pulses, B)determining a rate of change of pulse energy with charging voltage,ΔE/ΔV, C) controlling the pulse energy of each pulse P_(N) in the firstK pulses, in said burst of pulses by regulating the charging voltage ofthe laser utilizing a computer processor programmed with a firstalgorithm which: (1) determines for each P_(N) a pulse energy error, ε,based on a measured energy of at least one previous pulse in said burstand a predetermined target pulse energy value, (2) determines for eachP_(N) an integrated dose error, D, of all previous pulses P₁ throughP_(N−1) in said burst, (3) determines a charging voltage V_(N), for eachof said pulses, P_(N), in said first plurality of pulses using: (i) saidΔE/ΔV (ii) said ε (iii) said D (iv) a reference voltage based onspecified voltages for PN in a plurality of previous bursts, D)controlling the pulse energy of each pulse P_(K+N), in pulses followingP_(K) in said burst of pulses by regulating the charging voltage of thelaser utilizing a computer processor programmed with a second algorithmwhich: (1) determines for each P_(N) a pulse energy error, ε, based on ameasured energy of at least one previous pulse in said burst and apredetermined target pulse energy value, (2) determines for each P_(N)an integrated dose error, D, of all previous pulses, P₁ through P_(N−1)in said burst, (3) determines a charging voltage, V_(N), for each ofsaid pulses, P_(N) in said first plurality of pulses using: (i) saidΔE/ΔV (ii) said ε (iii) said D (iv) a reference voltage based onspecified voltages for P_(N) in a plurality of previous bursts, whereinsaid V_(N)'s in said fast and second algorithms are functions of atleast said ΔE/ΔV, said ε, said D and said at least one reference voltageand said V_(N)'s when calculated one utilized to adjust said chargingvoltage to control both the individual pulse energy and the integratedenergy dose to desired values.
 29. A laser as in claim 1 and furthercomprising an anode support means comprising a tapered surface forreducing the magnitude of aerodynamic reaction forces resulting fromlaser gas exiting said blower and being redirected by said anode supportmeans.
 30. A laser as in claim 1 and further comprising a fluorineinjection system comprising a processor programmed with an algorithmdesigned to cause fluorine to be injected continuously or at intervalsof less than 30 minutes in order to maintain fluorine concentrationsubstantially constant at a desired concentration over extended timeperiods of at least several days.
 31. A laser as in claim 30 whereinsaid fluorine injection system and further comprising a feedbackproviding to said processor a voltage signal representative of laserdischarge voltages which signal is used by said processor to maintainsaid signal within a predetermined range.
 32. A laser as in claim 31wherein said predetermined range is revised periodically in order tokeep the laser operating with a fluorine concentration within a desiredrange.
 33. A laser as in claim 32 further comprising a means forperiodically determining a laser parameter representative of a temporalpulse width of the laser pulses.
 34. A laser as in claim 30 wherein saiddetermined parameter is the integral square pulse width.
 35. A reliablemodular production quality high repetition rate excimer laser forproducing a narrow band pulsed laser beam at a repetition rate of atleast about 1Khz, said laser comprising: A. a quickly replaceable laserchamber module comprising: 1) two elongated electrodes 2) a laser gascomprised of fluorine and a buffer gas, 3) a gas circulator system forcirculating said laser gas between said electrodes at at least twocm/millisecond comprising: a) a braze-free blade structure defining ashaft, b) a brushless motor for rotating said shaft, c) magneticbearings for supporting said shaft said motor and said bearings havingrotors attached to said shaft and sealed within an environment exposedto said laser gas and said motor and said bearings having a statoroutside of said laser gas environment. B. a pulse power systemsubstantially contained within at least one quickly replaceable moduleand comprising: 1) a processor controlled high voltage power supply forperiodically, at rates of at least about 1000 Hz, charging withelectrical energy a charging capacitor to a predetermined pulse controlvoltage, 2) a compression and amplification circuit for connectingelectrical energy stored on said charging capacitor into a high voltageelectrical pulses of at least 14,000 volts across said electrodes.
 36. Alaser as in claim 35 wherein said chamber and said gas circulator definea gas flow path and an upstream direction and said laser also comprisesa single preionizer tube located upstream of said electrode and whereineach of said electrodes define an electrode length and said singlepreionizer tube is comprised of a grounded electrically conducting rodpositioned along the axis of an Al₂O₃ hollow cylindrical tube having alength longer than said electrode length.
 37. A laser as in claim 35 andfurther comprising a means of measuring the rate of change of pulseenergy with changing voltage ΔE/ΔV, and a computer controller programmedwith an algorithm for controlling pulse energy and integrated energydose in a burst of pulses defining present burst pulses, P₁, P₂ . . .P_(N) . . . P_(k),P_(K+1), P_(K+2) . . . P_(K+n), . . . P₁, P₂ . . .P_(N−1), P_(N), from said laser having a pulse power system including ahigh voltage charging system defining a charging voltage, said algorithmcomprising the steps of: A) measuring the energy in each pulse of saidburst of pulses, B) determining a rate of change of pulse energy withcharging voltage, ΔE/ΔV, C) controlling the pulse energy of each pulseP_(N) in the first K pulses, in said burst of pulses by regulating thecharging voltage of the laser utilizing a computer processor programmedwith a first algorithm which: (1) determines for each P_(N) a pulseenergy error, ε, based on a measured energy of at least one previouspulse in said burst and a predetermined target pulse energy value, (2)determines for each P_(N) an integrated dose error, D, of all previouspulses P₁ through P_(N−1) in said burst. (3) determines a chargingvoltage V_(N), for each of said pulses, P_(N), in said first pluralityof pulses using: (i) said ΔE/ΔV (ii) said ε (iii) said D (v) a referencevoltage base on specified voltages for PN in a plurality of previousbursts, D) controlling the pulse energy of each pulse P_(K+N), in pulsesfollowing P_(K) in said burst of pulses by regulating the chargingvoltage of the laser utilizing a computer processor programmed with asecond algorithm which: (1) determines for each P_(N) a pulse energyerror, ε, based on a measured energy of at least one previous pulse insaid burst and a predetermined target pulse energy value, (2) determinesfor each P_(N) an integrated dose error, D, of all previous pulses P₁through P_(N−1) in said burst, (3) determines a charging voltage, V_(N),for each of said pulses, P_(N) in said first plurality of pulses using:(i) said ΔE/ΔV (ii) said ε (v) said D (vi) a reference voltage based onspecified voltages for P_(N) in a plurality of previous bursts, whereinsaid V_(N)'s in said first and second algorithms are functions of atleast said ΔE/ΔV, said ε, said D and said at least one reference voltageand said V_(N)'s when calculated one utilized to adjust said chargingvoltage to control both the individual pulse energy and the integratedenergy dose to desired values.